When I bought my first DP,(DP1)I used the ertyl 18th scale display case to house it, although I must admit, as I wanted to display it on the DP base, I used a skill saw and cut the base lower, so it would all fit in and close tight.
These cases are only 5 inches high, so when I got the philly I had to buy the minichamps one as it was a full inch higher, but over here in the UK it cost me FORTY BUCKS !!!!!
Of course the 5 inch high one is adequate for the new E21, if you dont want to include the DP base, anf for about $10 you can get this, and a multitude of others from diecastdirect.com, and use the search feature for display case.
Most of them have the dimentions, some dont, but the 13 X 6 X 5 seems the best option, for just a basic FDNY pumper not on its base.

Location: London UK

Sorry about the bad flash flare in the pic.
The top one is the 5 inch high case, with a cut down DP base, a tight fit, but the 2 lower cases, even with the boom on the philly fit well, these were the 6 inch high minichamps ones.

Location: London UK